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Scattering of Coastal-Trapped Waves by Irregularities in Coastline and Topography

John L. WilkinJoint Program in Oceanographic Engineering, MIT/WHOI. Woods Hole, Massachusetts

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David C. ChapmanWoods Hole Oceanographic Institution, Woods Hole, Massachusetts

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Abstract

The scattering of freely-propapting coastal-trapped waves (CTWs) by large variations in coastline and topography is studied using a numerical model which accomodates arbitrary density stratification, bathymetry and coastline. Particular attention is paid to the role of stratification which in moderate amounts can eliminate backscattered free-waves which occur. theoretically, in a barotropic ocean.

Numerical simulations using widening and narrowing shelf topographies show that the strength of the forward scattering into transmitted CTW modes is proportional to a topographic warp factor which estimates the severity of the topographic irregularities. The forward-scattering is further amplified by density stratification. Within the scattering region itself, the strengths of the scattered-wave-induced currents exhibit substantial variation over short spatial scales. There is generally a marked intensification of the flow within the scattering region, and rapid variations in phase. On narrowing shelves, the influence of the scattering can extend upstream into the region of uniform topography even when no freely-propagating backscattered waves exist.

A simulation is conducted of CTW scattering at a site on the East Coast of Australia where observations suggest the presence of scattered freely-propagating CTWs. The success of the model simulation in reproducing, features of observations supports the notion that realistic shelf geometries can matter significant levels of CTW energy, and that the watered waves can have an appreciable signal in current-meter observations made on the continental shelf. This suggests that along irregular coastlines, it is important to account for the possibility that CTW scattering may be occurring if oceanographic observations are to be interpreted correctly.

Abstract

The scattering of freely-propapting coastal-trapped waves (CTWs) by large variations in coastline and topography is studied using a numerical model which accomodates arbitrary density stratification, bathymetry and coastline. Particular attention is paid to the role of stratification which in moderate amounts can eliminate backscattered free-waves which occur. theoretically, in a barotropic ocean.

Numerical simulations using widening and narrowing shelf topographies show that the strength of the forward scattering into transmitted CTW modes is proportional to a topographic warp factor which estimates the severity of the topographic irregularities. The forward-scattering is further amplified by density stratification. Within the scattering region itself, the strengths of the scattered-wave-induced currents exhibit substantial variation over short spatial scales. There is generally a marked intensification of the flow within the scattering region, and rapid variations in phase. On narrowing shelves, the influence of the scattering can extend upstream into the region of uniform topography even when no freely-propagating backscattered waves exist.

A simulation is conducted of CTW scattering at a site on the East Coast of Australia where observations suggest the presence of scattered freely-propagating CTWs. The success of the model simulation in reproducing, features of observations supports the notion that realistic shelf geometries can matter significant levels of CTW energy, and that the watered waves can have an appreciable signal in current-meter observations made on the continental shelf. This suggests that along irregular coastlines, it is important to account for the possibility that CTW scattering may be occurring if oceanographic observations are to be interpreted correctly.

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